You've got a fan rated at 300 cubic feet per minute. Your simulation software wants the number in cubic inches per second. Suddenly, a simple spec sheet turns into a math problem nobody warned you about. This guide fixes that. In the next few minutes, you'll understand exactly how these two units relate, why engineers switch between them, and how to convert any value in seconds.
Two Units, One Job: Measuring Volume Flow Rate
Both cubic feet per minute and cubic inches per second belong to the same family of measurement: volume flow rate. That's the measure of how much space a fluid or gas fills as it moves past a point during a set amount of time.
Think of water pouring from a tap. Volume flow rate answers two questions at once: how much water comes out, and how fast it arrives. The units just change the "ruler" and the "clock" used to describe that movement.
What Is Cubic Foot Per Minute (ft³/min)?
A cubic foot per minute describes the volume of one cube measuring one foot on every side, delivered over sixty seconds. It's the workhorse unit for moving air at room scale. If you've ever shopped for a bathroom exhaust fan, an air compressor, or a range hood, you've seen this unit listed on the box.
In everyday conversation, people often call it CFM. It's popular because a cubic foot is large enough to describe household and industrial airflow without huge, unwieldy numbers.
What Is Cubic Inch Per Second (in³/s)?
A cubic inch per second describes a much smaller cube, one inch on each side, delivered every single second. Because both the volume and the time window shrink, this unit zooms in on detail.
That makes it a favorite wherever precision matters more than bulk: hydraulic circuits, fuel delivery, laboratory gas sampling, and small pump testing. When your system moves modest volumes and reacts quickly, cubic inches per second gives you numbers that feel natural instead of tiny fractions.
How the Two Units Connect
The bridge between these units rests on two simple facts you probably learned in school:
- Length: One foot equals twelve inches. Cube that, and one cubic foot holds 12 × 12 × 12 = 1,728 cubic inches.
- Time: One minute contains sixty seconds.
So imagine one full cubic foot arriving over the course of a minute. Spread those 1,728 cubic inches across sixty seconds, and each second carries 1,728 ÷ 60 = 28.8 cubic inches. That's the heart of the conversion.
The Formula Worth Remembering
To go from cubic feet per minute to cubic inches per second, multiply by 28.8:
in³/s = ft³/min × 28.8
Going the other direction? Divide by 28.8. That single number handles both trips, because it bundles the size difference (1,728×) and the time difference (÷60) into one step.
A Worked Example
Say your ventilation plan calls for 250 ft³/min, but your duct simulation tool expects in³/s. Multiply 250 by 28.8 and you get 7,200 in³/s. No calculator gymnastics, no guesswork.
Why This Conversion Shows Up in Real Work
HVAC and Building Ventilation
Fan manufacturers speak fluent CFM. Simulation software, duct calculators, and some international documentation prefer smaller, per-second units. Converting keeps everyone on the same page when a design moves from a catalog page into an engineering model.
Pneumatics and Compressed Air
Air cylinders, actuators, and pneumatic tools often get sized using airflow figures. Component datasheets may list consumption in cubic feet per minute, while valve flow tests report results in cubic inches per second. Matching those numbers prevents undersized valves and sluggish equipment.
Hydraulics and Fluid Power
Hydraulic systems deal in precise, repeatable volumes. Pump displacement and cylinder speed calculations frequently start in cubic inches because cylinder bores and strokes are measured in inches. Converting a pump rating from ft³/min puts everything into one consistent language.
Laboratories and Instrumentation
Gas analyzers, sample pumps, and calibration rigs move small volumes quickly. Technicians comparing a blower's brochure rating against a flow meter's readout need this conversion to confirm the two devices actually agree.
Common Mistakes (and How to Dodge Them)
- Squaring instead of cubing. Twelve inches per foot becomes 1,728 cubic inches per cubic foot only when you cube it. Using 144 (twelve squared) throws every result off badly.
- Forgetting the time shift. Changing feet to inches isn't enough. You must also stretch a minute into sixty seconds, which is why the final factor is 28.8 rather than 1,728.
- Rounding too early. Keep full precision through your calculations, then round the final answer. Early rounding compounds errors across multi-step designs.
- Confusing volume flow with mass flow. Some ratings describe weight moved per unit time, not volume. Check the label before converting; the two are related but not interchangeable.
Quick Reference Table: ft³/min to in³/s
Here are the most commonly needed values, ready to copy straight into your notes or specifications.
| Cubic Feet Per Minute (ft³/min) | Cubic Inches Per Second (in³/s) |
|---|---|
| 1 | 28.8 |
| 2 | 57.6 |
| 5 | 144 |
| 10 | 288 |
| 20 | 576 |
| 50 | 1,440 |
| 100 | 2,880 |
| 500 | 14,400 |
Notice the pattern: every value scales cleanly by 28.8. Once that multiplier clicks, you can convert anything on the fly, even values that never appear on a chart.
Frequently Asked Questions
Is the 28.8 factor exact?
Yes. It comes directly from fixed definitions: twelve inches in a foot and sixty seconds in a minute. There's no approximation hiding inside it, so your converted values stay exact.
Which unit should my project use?
Follow the context. Large-scale air movement reads naturally in ft³/min, while compact, high-precision systems suit in³/s. When collaborating, match whatever unit your team's drawings, software, or standards already use.
Does temperature or pressure change the conversion?
No. This conversion is purely geometric and time-based, so it holds regardless of operating conditions. Just remember that some ratings specify air at particular reference conditions, so always compare like with like when evaluating equipment.
The Takeaway
Cubic feet per minute and cubic inches per second measure the same thing, volume flow rate, at different scales. One multiplication by 28.8 moves you between them with total confidence. Whether you're sizing a ventilation system, checking a compressor datasheet, or validating a lab instrument, that single number turns a frustrating mismatch into a five-second fix.
Ready to convert a value that isn't on the chart? Scroll up, enter your number, and let the tool handle the math while you get back to the project that matters.
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